While the high density, high melting point and strength of tungsten alloys make them a good candidate material for use in kinetic energy penetrators and other high stress applications, there has been a need for improved performance of tungsten alloys for use in such applications. Commercially pure tungsten is relatively brittle but it is known in the art that alloying tungsten with nickel-copper or nickel-iron binders can produce two phase alloys with useful strength and ductility for these applications. It is also known that the composition ratio of alloying additions to tungsten must be kept within a range that avoids formation of intermetallic compounds which causes embrittlement in the alloy. For example, in the commercially important tungsten-nickel-iron alloy system the nickel to iron ratio is generally held within the range of about 1:1 to 4:1. Outside this range, brittle, intermetallic phases form in the binder phase which rapidly degrade the properties.
The propensity of the iron and nickel alloys to form an intermetallic phase is well known in the art. At low nickel to iron ratios the intermetallic Fe.sub.7 W.sub.6 is known to form as a .mu. phase. Above the ratio of about 4:1 nickel to iron, a series of Ni:W intermetallics can form, including Ni.sub.2 W, NiW and Ni.sub.4 W. Heat treatments can be used to effectively break down such intermetallics since they are not stable at temperatures above 1000.degree. C. Quenching from a solutionizing temperature of about 1050.degree. C. can retain the ductile austenitic binder or matrix in the two phase system.
At the iron rich end of the alloy composition, the .mu. phase, Fe.sub.7 W.sub.6, is stable to a temperature of about 1640.degree. C., which is above the normal temperature range for sintering these alloys. The .mu. phase can only be controlled by diffusion into the austenite within a narrow temperature range corresponding to a limited tungsten solubility.
It has been well established that the tendency to the formation of the topologically close packed .mu. (mu) or sigma phases or intermetallics can be determined by calculating an electron vacancy number, N.sub.v, for a given composition of the alloys. For instance, the N.sub.v of the heavy alloy binder is related to the chemistry of the binder by the following equation: ##EQU1## where %Ni, %Co, %Fe and %W refer to their concentrations in the binder phase expressed in atomic %. The multiplicity factor assigned to each element (such as 0.66, 1.66, . . . etc.) indicates the propensity of the element to the formation of the intermetallic phase. If the N.sub.v value for a given binder composition exceeds a critical value C* (the actual value of C* is dependent on the amount of tungsten in solution, temperature and a constant for the particular alloy system), then the binder is susceptible to the formation of intermetallic phases. If the Nv value of the binder alloy is less than C*, then it is free from intermetallic formation.
Based on the above criterion, it is clear that elements which have a higher multiplicity factor would be more prone to the formation of the intermetallic phase compared to an element which has a lower multiplicity factor. For instance, nickel has the lower value (0.66) and, therefore has the least propensity to intermetallic formation compared to iron (2.66) or tungsten (4.66). Substitution of cobalt (1.66) for nickel would tend to raise the N.sub.v value and make the alloy more susceptible. Therefore, it would not be expected that replacement of nickel by cobalt decreases the formation of intermetallics and, therefore, improves the mechanical properties of the resulting alloys. While it is known that cobalt additions to the tungsten-nickel-iron system increase strength and hardness, as the amount of cobalt is increased embrittlement of the sintered alloy is also increased. Furthermore, when such cobalt-containing sintered alloys, and particularly those containing large amounts of cobalt in the binder are subsequently subjected to an annealing treatment, embrittlement of the material occurs, making it virtually useless for its intended purpose in high stress applications, such as kinetic energy penetrators.
Thus U.S. Pat. No. 2,793,951 discloses a powder metallurgical process for producing dense tungsten alloys wherein the main constituent consists of tungsten and/or molybdenum and a minor constituent consisting of one or more of the metals iron, nickel, cobalt, chromium with the proportion of the main constituent being not less than 75% by weight of the alloy. The alloys are made by sintering compacted mixtures of the metal powders in the requisite proportions. The inclusion of chromium in the alloy results in improving the hardness of the alloy.
U.S. Pat. No. 3,254,995 discloses heavy metal alloys having relatively high tungsten content and having high density, high tensile strength and high elongation properties, wherein the core of the alloy has substantially as good properties as the outside surfaces. Such properties are enhanced due to the use of iron in substantially equal or greater proportion than the nickel. The addition of small amounts of cobalt to the tungsten-iron-nickel alloy increases the sintering temperature range and stabilizes the part during sintering. It is stated that the cobalt additions do not impair the properties and may even slightly enhance them. Cobalt may be used effectively in amounts up to 1% of the total weight of the alloy. While higher amounts of cobalt may be used, for most applications about 1% or less has been found adequate. The alloys are produced by sintering in a hydrogen atmosphere and then cooled.
U.S. Pat. No. 3,988,118 discloses tungsten alloys containing minor amounts of nickel, iron and molybdenum and at least one additional element which either increases the mechanical properties at room temperature, including strength, ductility and/or increases the corrosion resistance and resistance to oxidation at elevated temperatures and/or increases the resistance to thermal fatigue. These additions include cobalt, chromium, manganese, vanadium, tantalum, zirconium, titanium, yttrium, rhenium, boron and silicon. Cobalt is said to inhibit the formation of undesirable intermediate compounds, such as tungsten and nickel, and should be used in the range of about 0.5 to 5% by weight percent. Heat treating the sintered compact in a neutral or slightly reducing atmosphere and then quenching rapidly produces elongations of from 5 to 25% in the treated alloy.
U.S. Pat. No. 4,012,230 discloses a tungsten-nickel-cobalt alloy and a method for making such alloy wherein tungsten particles are coated with a nickel-cobalt alloy, compacted to shape, heated in hydrogen to 1200.degree. to 1400.degree. C. for one hour and cooled to about 1200.degree. C. The hydrogen atmosphere is then replaced by argon and the shaped sintered compact is held at that 1200.degree. C. temperature for one half hour and is then cooled to room temperature in the argon atmosphere. The patent states that considerable hardness occurs in these alloys at lower sintering temperature. The alloys show high strengths and can have good ductilities. Use of two percent cobalt in the alloy is disclosed.
It is known in the art that the strength and hardness of tungsten-nickel-iron alloys can be increased by imparting some degree of work to such alloys. For example, swaging a sintered bar by a reduction in cross-sectional area of 25% can increase the hardness of a 93% W-4.9% Ni-2.1% Fe tungsten alloy from 30 points on the Rockwell C scale of hardness to about 38-40 points. It is also a known characteristic of these alloy systems that they strain age readily at modest temperatures after introduction of pre-strain by working.
In a paper entitled "Studies of Tungsten Heavy Metals" by G. Jangg, R. Keiffer, B. Childeric and E. Ertl appearing in Planseeberichte fur Pulvermetallurgie 22 (1974), 15-28, the authors disclose that a small cobalt addition to tungsten heavy metal alloys containing nickel and iron has a positive effect on ductility and hardness of the alloy when compacts of such alloys are sintered. The values of density, hardness and torsional fracture angle are a function of the sintering temperature and sintering time with such temperature being 1460.degree. C. and the time being about 60 minutes for a 90.8 W-5.5 Ni-2.8 Co-1.9 Fe alloy. The article states that hardness is more greatly affected by variation of binder composition and concludes that toughness and hardness of the alloy are not affected in entirely the same way and that a favorable combination of good hardness values with a high torsional angle can be chieved with a binder composition of 50 to 55% Ni, 25 to 30% Co and 20% Fe. While the authors disclose that in the as-sintered condition, the W-Ni-Fe-Co alloys are superior to the conventional W-Ni-Fe alloys, they do not teach how such as-sintered properties can be further improved.
In a paper entitled "Effects of Cobalt on Nickel-Tungsten Alloys," by F. F. Schmidt, D. N. Williams and H. R. Ogden, Cobalt, 45, December 1969, at pages 171-176, inclusive, the effect of cobalt on the mechanical and metallurgical properties of nickel-tungsten alloys wherein the alloys contained 45 or 50 percent tungsten is discussed. However, the tungsten-nickel-cobalt alloys which were formed are single phase austenites in which all of the individual ingredients have been dissolved to form the alloys. The systems disclosed in this paper are entirely different from the high density system of the present invention.